Various devices for distributing foamable reaction s mixtures upon moving substrates are known. Such devices are used in the production of rigid polyurethane and/or polyisocyanurate foam boards and metal-faced panels. Typical of the known devices are traversing pipes, stationary multi-stream pipes and stationary spreader bars having spray nozzles. One recurring problem with known equipment is that the foamable reaction mixture cannot be distributed over the entire moving substrate with the result that the foam being formed is forced to flow and meld together, causing the formation of knit lines, irregular cell orientation and voids or air pockets. In addition, existing devices are frequently plugged with reaction mixture after a relatively short time causing shut down of the equipment until the plugging can be removed. Typical of the prior art devices are those described in U.S. Application Ser. No. 236,061, filed on August 24, 1988 now U.S Pat. No. 4,900,593 and No. 156,775, filed on February 16, 1988, now U.S. Pat No. 4,846,099.